Hydrophobic Monomer Incorporation Tunes the pH Responsivity of Polycationic Brushes

Abstract

We tune the pH-responsive swelling response of polycationic brushes via the incorporation of hydrophobic repeat units. Careful compositional tuning realizes apparent brush pKa values, which we term pKa*, between 5.0 and 5.7. Further, we observed monotonic decreases in pKa* with an increase in hydrophobic monomer incorporation. Although brush pKa* was readily tuned with composition, the cooperativity of deprotonation was influenced only by polycation identity and not hydrophobic monomer incorporation. Interestingly, hydrophobicity-driven pKa* decreases were observed exclusively for brushes and not untethered, compositionally matched polymers, suggesting that confining polymers in the form of nanoparticle-tethered brushes might grant more fine-tuned control over pH-responsivity than the tunability offered by free polymers alone. We readily reproduced pKa* trends from planar substrates to nanoparticle-tethered brushes, potentiating applications in intracellular pH sensing and nucleic acid delivery that demand precise pKa tuning. We will apply pKa*-tuned brush-grafted nanoparticles to overcome intracellular barriers that have previously hindered free polymers.